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Date: Wed, 11 Nov 2009 12:00:17 +0100 From: Corrado Zoccolo <czoccolo@...il.com> To: Przemysław Pawełczyk <przemyslaw@...elczyk.it> Cc: lkml <linux-kernel@...r.kernel.org> Subject: Re: SATA NCQ - blessing for throughput, curse for latency? Hi, 2009/11/11 Przemysław Pawełczyk <przemyslaw@...elczyk.it>: > Hi, > > [snip] I wrote a script (that is pasted > at the end), which using zcav from bonnie++ (and unbuffer from expect > to work properly if output is redirected to a file) reads up to 64 GB > of the disk, but after a first few gigabytes (depending on how many GB > of memory you have) cats kernel sources with binary files up to the > second level of directory depth. What is measured? The real time of > the cat to /dev/null operation. Why not the kernel sources alone? > Because if we build kernel, especially with -jX (where X > 1), then > both inodes and contents of the corresponding files within these > directories generally won't be consecutive. Time for the results with > some background information. In my tests I have used 2.6.31.5 built > with debuginfo (using debian's make-kpkg). > >[summarized the relevant numbers] > *** NCQ turned on *** > real 752.30 > user 0.20 > sys 1.83 > > > *** NCQ turned off *** (kernel booted with noncq parameter) > real 62.30 > user 0.27 > sys 1.59 > > [snip] > > Can anyone confirm analogous NCQ-dependant behavior with other > disk/controller variants? Any suggestions for improvements other than > turning off NCQ or switching to anticipatory (in typical cases cfq is > better, so it's not the best option)? The NCQ latency problem is well known, and should be fixed in 2.6.32. You can try building the latest rc. -- __________________________________________________________________________ dott. Corrado Zoccolo mailto:czoccolo@...il.com PhD - Department of Computer Science - University of Pisa, Italy -------------------------------------------------------------------------- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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